They generally include a moon phase mechanism, formed of a gear train driven by an hour wheel and of a disc to which two representations of the moon are affixed. The disc is visible behind an aperture in the general shape of a semi circle, the diameter side of which includes two convex portions arranged such that the moon appears to be ascending, full, then descending progressively during its path through the aperture. This conventional representation of the phases of the moon does not permit any fun or surprising effects, although such effects are greatly sought after in horology. Moreover, this representation is not true to reality, since the incline of the moon relative to the 12 o'clock-6 o'clock axis of the watch, varies depending upon the position of the moon in the aperture. When a face is drawn on the moon, as is commonly the case, at first the face is perpendicular to the 12 o'clock-6 o'clock axis, then parallel, then perpendicular again. The effect obtained is neither realistic nor attractive.
The present invention overcomes these drawbacks by proposing a timepiece fitted with a moon phase indication wherein the representations of the moon are mobile in rotation on the moon phase disc.